The invention relates to a close formation flight positioning system using air data measurements. Military aircraft fly in formation for a variety of reasons. In some of these formations, the aircraft are sufficiently close to one another that the trailing wake from the lead aircraft affects the aerodynamic characteristics of the follower aircraft. This situation is commonly termed “close formation flight”. One example of close formation flight is aerial refueling, where a receiver aircraft flies behind and below a tanker aircraft. Close formation flight can also be used for drag reduction, with follower aircraft flying in the beneficial upwash generated by the leaders, as is common in migratory birds.
It is very difficult for pilots to maintain the proper position for long intervals of close formation flight due to the high pilot workload. To rectify this, some sort of automatic positioning system is desirable. Such a system must be able to determine the relative locations of the aircraft to a high degree of accuracy (within feet), to avoid the possibility of a collision. Unmanned aircraft in close formation flight also require such a system.
The present invention uses two or more air data sensors on the follower aircraft, located several feet from one another in either the lateral or vertical direction. The air data sensors must be of the type that can measure the impact pressure (and resultant horizontal velocity), the angle of attack, and the angle of sideslip, such as the system disclosed by Hagen (U.S. Pat. No. 4,836,019). These measurements can be converted to horizontal, vertical and lateral velocity components.
The wake vortex system of the lead aircraft generates horizontal, vertical and lateral induced velocities that vary with horizontal, vertical and lateral position from the wing. By comparing the velocity component measurements from the sensors of the follower aircraft, using the known distance between the sensors on the follower aircraft and the wing span of the lead aircraft, the distance from the follower aircraft to the lead aircraft can be determined. If the conditions of the lead aircraft (weight, speed and altitude) are known, then two sensors on the follower aircraft will be sufficient to determine the relative positions. If the conditions of the lead aircraft are not known, then three sensors on the follower aircraft will be sufficient to determine the relative positions.
There are various means for determining relative position of aircraft in a formation, each of which has disadvantages.
U.S. Pat. No. 4,763,861, Microwave Rendezvous System for Aerial Refueling, issued to Newman, discloses a system that transmits microwave signals from the lead aircraft (tanker) to the follower aircraft, which processes these signals to determine the relative position. This system requires equipment to be installed on all of the aircraft in the formation, and is thus not a completely self contained system.
U.S. Pat. No. 5,904,729, Automatic Director Light System for Aerial Refueling Operations, issued to Rizicka, discloses a system that uses a 3-D camera on a follower aircraft which in combination with stored 3-D images of the lead aircraft and a data processor, determines the position of the lead aircraft relative to the follower aircraft. This system cannot be used if the aircraft are in clouds or if the field of view of the camera is blinded by the sun, so it is not an all weather system.
U.S. Pat. No. 5,906,336, Method and Apparatus for Temporarily Interconnecting an Unmanned Aerial Vehicle, issued to Eckstein, discloses a system that transmits either electro-optical, radar or infrared signals from the lead aircraft or an object towed from it to the follower aircraft. This system requires equipment to be installed on all of the aircraft in the formation, and is thus not a completely self contained system.
U.S. Pat. No. 6,963,291, Dynamic Wake Prediction and Visualization with Uncertainty Analysis, issued to Holforty et al, 2005, discloses a system that predicts the location and movement of trailing wakes of other aircraft using a combination of inertial navigation system and air data system measurements on the vortex generating aircraft. The purpose of this system is to predict the long term position of wakes so they can be avoided by follower aircraft. It uses accepted mathematical models to represent the wake. This system uses either GPS (Global Positioning System) or ADS-B (Automatic Dependent Surveillance Broadcast) signal to determine the position of the vortex generating aircraft. This system requires a data link between lead and follower aircraft (or between the aircraft and the ground) so it is subject to jamming.
U.S. Pat. No. 6,963,795, Vehicle Position Keeping System, issued to Hassig et al, discloses a system that uses a data link between the leader and follower vehicles in the formation. The follower vehicles use their velocity and angular rate information to determine guidance corrections to maintain position. This system requires a data link between lead and follower aircraft (or between the aircraft and the ground) so it is subject to jamming.
U.S. Pat. No. 7,024,309, Autonomous Station Keeping System for Formation Flight, issued to Doane et al, discloses a system that uses either GPS or millimeter wave radar signals. The sensors and computational algorithms of this system allow the relative position of the aircraft to be determined within about 1 meter. This system requires equipment to be installed on all of the aircraft in the formation, and is thus not a completely self contained system. The system also requires a data link between the aircraft so it is subject to jamming.
Some advantages of the present invention are that it is a totally self contained system that does not require a data link to any other aircraft or to the ground and, therefore, is not subject to jamming and it requires sensors only on the follower aircraft, thereby enabling a cost savings. Furthermore, the present invention can be used in all weather operations including cloudy or foggy weather.